1. Field of the Invention
The present invention relates to an image processing device and method, and in particular, to an image processing device which converts image data such that saturation of an image is changed, and to an image processing method which can be applied to the image processing device. Further, the present invention relates to an image processing method which can convert the gradation of image data, and to an image processing device to which the image processing method can be applied.
2. Description of the Related Art
In research and development of photosensitive materials such as photographic films, photographic printing papers and the like, it is known that, in order to realize vivid color formation of a photographed image, when a photosensitive material is exposed by using a monochromatic light whose main light is a specific color component (RGB), the developing suppressing effect from other color photosensitive layers weakens (the so-called interlayer effect) as compared to a case in which exposure is carried out by using white light. As a result, the photosensitive layers must be designed such that the saturation of the regions corresponding to chromatic photographed subjects is reproduced even higher than the saturation in regions corresponding to photographed subjects in the photographed image which are achromatic or nearly achromatic.
In images of landscapes or objects at rest, by increasing the saturation of chromatic photographed subjects throughout the entire image as described above, the image has an apparently vivid finish. However, for images which include humans as photographed subjects, when the saturation of skin-colored regions corresponding to the skin of persons in the image is increased more than needed due to the increase in the saturation of the entire image, a problem arises in that the photographed image does not have a preferable finish, for example, the redness of or pimples on a person's face or the like are emphasized more than needed. Thus, for image quality, there is a tradeoff between the saturation of the entire image and the saturation of the skin-colored regions.
On the other hand, in image processing carried out on digital image data, it is possible to relatively easily and freely adjust and change the color reproduction of the image. For example, in 3×3 matrix processing in which a 3×3 matrix is multiplied by each pixel for RGB 3-channel image data, if the diagonal elements of the 3×3 matrix are made to be values larger than 1 and the non-diagonal elements are made to be values smaller than zero, the saturation can be increased. If the diagonal elements of the 3×3 matrix are made to be values less than 1 and the non-diagonal elements are made to be values greater than zero, the saturation can be decreased. However, in this case as well, when the saturation of the entire image is increased, the skin-colored regions corresponding to the flesh of people in the image have a finish which is not preferable.
Further, Japanese Patent Application Laid-Open (JP-A) No. 6-124329 discloses a saturation changing circuit used in a structure in which, on the basis of an inputted color signal, a multiplication factor expressing the degree of enhancement of the saturation is determined by a saturation detecting circuit, and due to a saturation raising circuit multiplying the determined multiplication factor by the saturation signal, the saturation is raised. In the saturation changing circuit, a factor for skin-color judgement is set by the saturation detecting circuit. At a specific-color judging circuit, on the basis of the hue of the inputted signal and the factor set by the saturation detecting circuit, a judgement is made as to whether that hue falls within a range of skin color. In a case in which it is judged that the hue falls within the range of skin color, the multiplication by the saturation raising circuit (i.e., the raising of the saturation) is stopped.
However, in the aforementioned saturation changing circuit, the saturation is changed by turning the saturation raising by the saturation raising circuit on and off on the basis of whether the hue of the inputted signal falls within a range of skin color. Thus, when the saturation is changed by the saturation changing circuit, a problem arises in that the image quality deteriorates, such as the finish in the vicinity of the outer edges of regions at which the saturation is raised by the saturation raising circuit and regions at which the saturation is not raised is unnatural. Further, in the saturation changing circuit, when an attempt is made to suppress the raising of the saturation of a plurality of respectively different hues, it is necessary to provide a specific-color judging circuit for each of the hues for which raising of the saturation is to be suppressed, and a problem arises in that the structure of the device becomes complex.
Further, the color reproduction characteristics which are preferable at the time of reproducing an object, which is photographed by a camera, as an image on a recording medium differ in accordance with the type of the photographed object, the uses of the image, and the like. Thus, plural types of photographic photosensitive materials having respectively different characteristics are developed and produced as photographic photosensitive materials such as photographic films, photographic printing papers, and the like. For example, in a case in which photographing is carried out by a professional cameraman (especially photography at a photo studio or photography for portraiture) and photographic prints are prepared from the images recorded on the photographic film by this photographing, a photographic film or photographic printing paper is used which has a soft gradation (low contrast) characteristic by which it is difficult for over color and under color (color loss in gradation separation) to arise in the region corresponding to the face of a person in the image. On the other hand, for general photography or amateurs, vivid color prints are preferable, and therefore, photographic films or photographic printing papers having hard gradation (high contrast)characteristics are usually used.
Further, different types of photographic photosensitive materials are used for different types of photographed subjects, different applications of the images, and the like. For example, a photographer may select the photographic film and carry out photographing in accordance with his/her personal tastes or the type of the object to be photographed or the like. At the place of development to which the photographic film is brought and at which preparation of photographic prints is requested, for example, if the person placing the request is a professional cameraman, the preparation of the photographic prints may be carried out by using a photographic printing paper which has been developed for professional use (e.g., a photographic printing paper having a soft gradation characteristic).
However, in order realize such use of different types of photographic photosensitive materials for different types of photographed subjects or different applications of images or the like, it is necessary to prepare in advance plural types of photographic photosensitive materials having respectively different characteristics. Thus, the financial burden is heavy, and the work involved in properly using the different types of photographic photosensitive materials in accordance with the different types of photographed subjects, different applications of the images and the like is complex.
Thus, the image data, which is acquired by reading by photoelectric converting elements an image recorded on a photographic film, is converted in accordance with a gradation conversion characteristic which is determined in accordance with the type of the photographed subject, the application of the image, or the like. The image data which has been subjected to gradation conversion is used to record the image onto a photographic photosensitive material. In this way, photographic prints of a finish which corresponds to the type of the photographic object or the application of the image or the like can be prepared from one type or a few types of photographic photosensitive materials. However, when the gradation of the image data is converted, a problem arises in that an unintentional change in the image quality (especially a change in the saturation) arises.
As a related technique, JP-A No. 5-328132 discloses a digital image forming device such as a digital printer or a digital copier. When the user inputs a target gradation curve by an inputting section, an emitted light controlling section corrects the exposure data on the basis of the inputted gradation curve. Due to a color masking correcting section carrying out color masking correction in accordance with the input of the gradation curve, adjustment is carried out such that the (saturation and the like) of the output image does not change even if the gradation curve changes.
However, in the technique disclosed in the aforementioned publication, by carrying out matrix computation, color masking correction is realized, and the control of the hardness/softness of the gradation and the control of the increase/decrease in saturation can be made to correspond to each other. For example, when a gradation curve in which the saturation becomes high is selected, a masking factor which decreases the saturation is selected as the masking factor (matrix factor) used in color masking correction, and a change in the saturation can thereby be suppressed. The adjustment of the saturation change amount is carried out by adjusting the values of the non-diagonal elements of the masking factor.
Thus, in cases such as when a gradation curve whose slope (and in particular, slope from the highlight regions to the shadow regions) is not constant is selected, it is difficult to accurately correct the change in the image quality such as saturation or the like from the highlight regions to the shadow regions. A problem arises in that the accuracy of correction with respect to changes in image quality that accompany image gradation conversion is poor.